An easy way is to create a global variable that when a user purchases the no ads IAP, this variable is the changed.
So you could have the GV of noAds = 0
Incorporate this with you and code as an event stating that if noAds = 0 then ads will be displayed. If the user purchases the no ads, the global variable gets updated to 1. As long as the global variable is at 1, no ads will be displayed